It has always been a problem in the field of X-ray technique that certain parameters of exposure such as the anode voltage and current of the X-ray tube should be set when the X-ray generator is in the unloaded state i.e. the voltage is different from that under load and there is no current through the tube. The operator wishes to make sure that during the exposure the actual parameters be as closely equal to the preset values as possible. Further, the control of the X-ray generator should prevent the X-ray tube from being overloaded due to incorrectly set parameters.
The setting of the high voltage is generally done by using appropriate switches, allowing the voltage to be adjusted in course and fine steps. The unloaded high voltage V.sub.in is higher than the voltage V.sub.A during exposure due to the internal resistance of the X-ray generator. The load is constituted by the anode current I.sub.A which itself is a function of the voltage I.sub.A and of the filament current I.sub.F. Under unloaded conditions one can adjust the voltage V.sub.in and the filament current I.sub.F and this should be done in such a way that under loaded conditions the actual voltage V.sub.A and current I.sub.F correspond to the expected values. The exposure time is generally too short for establishing a closed regulation loop.
Apart from some very expensive types in the low and medium cost category of X-ray generators, voltage compensation is made in such a way that there are two switches moved together when the filament current is set and the second switch inserts respective resistors in series with a voltmeter measuring the unloaded voltage V.sub.in. The values of the resistors are chosen in such a way that in each position the voltage reading equals to V.sub.A instead of V.sub.in. The display of the expected voltage V.sub.A is of significance to the operator, who is generally an X-ray specialist.
The anode current I.sub.A cannot be set simply by means of the filament current I.sub.F, since with a given filament current the anode current increases with increasing voltage. In a widely used solution the filament is supplied from a source which includes a stabilized AC source often referred to as an isostat and a secondary winding of a separate transformer connected to the high-voltage AC lines, and this secondary winding was connected in series with the isostat, with reversed phase relative thereto, so that the resulting filament supply voltage was linearly decreased when the high voltage increased and vica versa. The discrete setting of the filament current occurred by means of serially connected resistors.
Substantially such a control can be found in the X-ray generator of U.S. Pat. No. 4,158,138 and in a number of types present nowadays in the U.S. market.